A. Field of the Invention
The catalytic cracking of hydrocarbons is performed in a tubular reactor wherein hydrocarbons to be processed are passed over particulate catalyst material to initiate the cracking process. Catalyst utilized in this process becomes contaminated with hydrocarbons and by-products of the cracking of hydrocarbons. Such catalyst is removed from the reactor, and, rather than being discarded, the catalyst is transferred to a regenerator. The catalyst can be regenerated in a fluidized bed regenerator vessel by subjecting the particulate catalyst to a oxidizing gas stream with release of heat. The regeneration process oxidizes carbon from the catalyst in order that the catalyst can be recycled through the hydrocarbon cracking reactor. The catalyst emanating from a fluidized bed regenerator vessel can be very hot and under high pressure. Such an environment is generally necessary for the regeneration process. The removal of regenerated catalyst under such conditions creates a problem of erosion due to pressure losses in the conduits emptying the regenerated catalyst. Various apparatus have been used to control such unloading of catalyst regenerators. Such apparatus have suffered from the attendant problems of apparatus erosion caused by the hot, reactive, particulate catalysts used in hydrocarbon cracking processes and the losses of fluidization in the regenerator catalyst beds due to equipment deterioration under such an erosive environment.
B. The Prior Art
The prior art has attempted to provide various solutions to the problems in recycling of cracking catalyst and its removal from regenerating vessels. These concepts have incorporated the use of various valves, piping arrangements and multiple flow paths.
In U.S. Pat. No. 3,342,561, Pohlenz et al., a catalytic hydrocarbon furnace is disclosed which has a circuitous flow path for catalyst discharging from the reactor portion of the furnace downwardly to the lower portion of a regenerator section. At that point, the regenerating catalyst is fluidized upwardly through the section and separated in a cyclone separator, before re-entering the furnace reactor. This patent fails to provide a reservoir for the catalyst and is dependent upon precise flow of the catalyst at all times. In addition, pressure conditions within the reactor are dependent upon the valve control of the carbonized catalyst return conduit to the regenerator section. Erosion problems are attendant with such a valve system.
In U.S. Pat. No. 3,904,548, Fagan et al., a hydrocarbon reactor and catalyst regenerator system is demonstrated, which provides a direct downward flow of regenerated catalyst to a reactor. The catalyst flows upward through the reactor and re-enters the regenerator by gravity. The catalyst flow from the regenerator to the reactor is controlled by a valve. Such a system fails to provide a separation of the pressurized, fluidized zones of the regenerator and reactor. In addition, the supply of catalyst to the reactor is dependent upon the operation of a valve, which is subject to the characteristically erosive conditions of hot fluidized catalyst.
In another patent, U.S. Pat. No. 3,964,876 to James, a regeneration system is disclosed which has a multiplicity of return conduits for providing regenerated catalyst to a vertical, tubular hydrocarbon reactor zone. The return conduits are designed with a downward gravity flow directly to the reactor zone, which is controlled by valves placed on the respective flow paths. Such a system requires that the control of catalyst flow depends entirely on typical valving which is subject to erosive conditions and provides only direct supply of catalyst material from the regenerator to the hydrocarbon reactor.
In U.S. Pat. No. 3,494,858 to Luckenbach, a regenerator-reactor system is provided which utilizes a catalyst settling zone and a downwardly curved conduit flow path for catalyst transported from the regenerator to the reactor and back again. The load of catalyst in this system is held in the downwardly flowing zone. Gas is added to the midpoint of this zone and not the bottom of this zone. This is done in order to maintain a high density of flowing catalyst so as to produce efficient regeneration thereof.
The present system by contrast is not a catalyst regeneration system as taught by Luckenbach but rather a continuous catalyst unloading system, i.e. a system which rejects the spent catalyst. As a result, the spent catalyst is not recycled, and there is no teaching in the Luckenbach patent that a special downstream pressure control is necessary in order to maintain an equilibrium between fresh catalyst loading and spent catalyst unloading as is necessary in the present invention. Presented in another way, the present system controls the pressure in a spent catalyst hopper in order to vent the carrier air, while settling the spent catalyst. This pressure control is manifested in conjunction with a novel upstream restriction zone, such that a steady catalyst unloading flow rate may be maintained. In addition, the present invention uses the restriction zone as a fluid seal during spent catalyst conveyance from one vessel to another. Also, the slow curved piping in the discharge conduit does not experience erosive wear due to the hot discharged catalyst, because of the refractory lining of the restriction zone. Thus, the restriction lining has a novel dual purpose which is not shown in Lukenbach.
Other prior art patents of general relevance to regenerated catalyst flow systems include U.S. Pat. No. 3,149,924, Cross, Jr.; U.S. Pat. No. 3,617,496, Bryson et al.; and U.S. Pat. No. 3,886,060, Owen.